Genetic and Environmental Effects on Disc Degeneration by Phenotype and Spinal Level A Multivariate Twin Study

Study Design. A classic twin study with multivariate analyses was conducted. Objective. We aimed to further clarify the presence and magnitude of genetic influences on disc degeneration, and to better understand the phenomenon of disc degeneration through comparisons of genetic and environmental influences on specific degenerative signs and different lumbar levels. Summary of Background Data. Previous studies suggest a substantial genetic influence on disc degeneration, but raise important questions about which disc phenotypes are or are not largely genetically influenced and differential effects on spinal levels. Methods. The study sample consisted of 152 monozygotic and 148 dizygotic male twin pairs, 35 to 70 years of age, from the population-based Finnish Twin Cohort. Lumbar magnetic resonance imaging was conducted with quantitative or qualitative assessments of disc signal, bulging, and height narrowing at each lumbar level. Data on possible confounding factors were obtained from an extensive, structured interview. Quantitative genetic modeling was conducted using MPlus. Results. Heritability estimates varied from 29% to 54%, depending on the particular disc degeneration phenotype and lumbar level. The same genetic influences affected signal intensity and disc height (genetic correlations of -0.60 - -0.66) or bulging (-0.71 - -0.72) to a great degree at either the lower or upper lumbar levels and genetic influences on disc height narrowing and bulging were virtually the same. (0.92-0.97). Conversely, genetic correlations (and environmental correlations) were substantially lower for upper and lower lumbar levels, implying largely independent effects. Conclusion. Genetic and environmental influences on disc degeneration seem to be of similar importance. Disc signal, narrowing, and bulging had a primarily common genetic pathway, suggesting a common genetic etiopathogenesis. Conversely, genetic and environmental influences differed substantially for upper versus lower lumbar levels, emphasizing the importance of examining these levels separately in studies of associated genes, other constitutional factors, and environmental influences.